SU960646A1 - Current active and reactive component measuring method - Google Patents

Description

The invention relates to a measurement technique and can be used to measure active or reactive currents in AC circuits containing a reactive load, in particular, to measure the active current of an induction motor.

The known method of measuring the passive component of the current, in which the constant signal is multiplied by the mains voltage, subtracts the resulting product from the total current, the resulting difference of the signals is cleaved by the mains voltage, sets the average value of the resulting product to zero, by adjusting the value of the constant signal one .

The disadvantages of this method are that the accuracy of signal processing is low due to the application of the multiplying operation of variable signals, which introduces significant errors in the obtained results, and the measurement performance is low, since the controlled constant signal is obtained from the variable by smoothing the last inertial device.

There is also known a method for measuring active-current, based on the fact that signals proportional to instantaneous values of current and voltage in a controlled circuit are fed to a key element. A signal proportional to the voltage of the network is used as a reference signal. Key unlocking and locking

10 elements are at random {at the moments when the reference signal passes through zero, due to the average load voltage in a period proportional to the amplitude of the active current 2.

15

The disadvantages of this known method are the error, which increases with the distortion of the sinusoidal shape of the monitored voltage and current, as well as when the symmetry of the monitored load is violated, which reduces the measurement accuracy, and the slow response rate, because the output signal has a pulsating shape, and to obtain an average over the 25th period of the voltage, the output signal needs to be smoothed.

The aim of the invention is to improve the accuracy and speed of measurement and to increase the speed to 30. The goal is achieved by the fact that according to the method, based on the use of signals proportional to the instantaneous values of voltage and current in the monitored circuit, fix the level of total current at the time when this level is equal to amplitude M5; the value of the measured component, and the moment of fixation time is determined once per period by the phase of the rectangular reference signal, the generated and measured voltage of the network. Moreover, the specified fixation of the total current level is made additionally a second time during the period, and the result and the measurements are inverted. The drawing shows a block diagram of a device implementing the proposed method. The device contains a shaper 1 of a reference signal (FOC), whose input is supplied with a signal proportional to the mains voltage, a control circuit 2 (SU), whose input is connected to the FOS output, two level locks ( FU1) 3 and (FU2) 4, the first inputs of which receive a signal and are proportional to the current, and the second inputs are connected to the output of the SU, and the summing element. (MF) 5, connected by its inputs to the outputs of the fixers levels. The output of the ESS is the output of the device. dnoco6 implement as follows. The voltage and, proportional to the instantaneous value of the total current measured in the circuit, goes to the first inputs of the level fixers. A pth TRC can be represented as the sum of its instantaneous values its component + lflr Ld5-1P UJtt3pSin (jUt). (1) where f is the phase shift between the mains voltage and current; LG JJJ cosH-amplitude value of the actin component; amplitude value of the reactive component. From formula (1), it follows that at the moment of time, when Ujt, .2, the reactive component of the complete one vanishes, and the active component acquires the maximum value. Consequently, at this moment of time, the magnitude of the total current corresponds to the amplitude value of the active component. Similarly, at the moment of time when ujt SG, the magnitude of the total current corresponds to the amplitude value; the reactive component, since the component is active at this moment is equal to zero. Level 3 and 4 tabs lock the instantaneous value of the U-J signal and store this signal value. The operating modes of the latches are controlled by control commands coming from the control circuit 2. In order for the –fixing of current levels to occur at times corresponding to a lot when measuring the active component or wt - J when measuring the reactive component, control circuit 2 generates control commands depending on the phase of the reference signal generated from the U signal {,, proportional to the voltage of the network. When the reactive component is measured, the signal Uc enters the shaper 1 of the reference signal, at the output of which a square-to-square signal is formed. Since Uc has a zero phase shift, then the time moment u) t L. corresponds to the transition of this signal through zero from a positive value to a negative one, and the control circuit 2 at this instant produces an equalizing command. With this command, latch 3 locks the signal level Ux, and then goes into storage mode, and at its output, a signal is received that is proportional to si to the peak value of the reactive component. By the same command, latch 4 goes from storage mode to latch mode and. at its output, the signal becomes zero. With the arrival through the period of the next control command at the moment. At the time when Iwt, the current level is locked in lock 4, which then goes into storage mode. As a result, the signal, proportional to the amplitude value of the reactive component, appears alternately with inter. a shaft in one period then at the output of latch 3, then at the output of latch 4. After summing the output signals at the output of summing element 5, a continuous signal appears that is proportional to the amplitude value of the reactive component of the current. Since the current level is fixed every period, the measurement speed is determined by the length of the measured waveform period. When measuring the active component of the current, the signal Uc is fed to the input of the former 1, at the output of which a rectangular reference form is formed, shifted relatively to the initial angle

-. Then MO.2. out - at some time when

2

corresponds to the zero transition of the reference signal from a negative value to a positive one, and the SU 2 generates a control command at this time. Further, the operation of the device occurs in the same way as when measuring the reactive component. Since the current is locked at times when

I

lot i

output summing

2

element 5, a signal appears that is proportional to the amplitude value of the active component of the current.

The accuracy of the measurement of the components of the current depends on the accuracy of detecting the fixing points of time and, since the reference signal has a rectangular shape, it is possible to obtain greater accuracy in determining the required points in time, which also leads to a high accuracy of measurement. In this case, the output signal does not have pulsations, which eliminates the need to use smoothing devices and allows to obtain a measurement response equal to one period of the measured signal.

A further increase in speed can be obtained by fixing the magnitude of the total current additionally at times, when

which iwt:

for measurement

the active component, and u) t 2jT for -1: from the meter of the reactive.

Stand. these values of u) t in formula (1) shows that the magnitude of the total current at given points in time is equal to the amplitude values of the components, but has a negative sign. Therefore, after inversion of the measured signal, the required information is obtained. Additional fixation points are detected by the phase of the same reference signals when they go through zero in the opposite direction, and in general the current is fixed using this measurement method twice during the reference signal period each time it passes through zero.

The drawing shows in dotted lines the place where a possible inverting amplifier (RI) 6 can be switched on in the block diagram.

The difference between the operation of the device and the previous one is that the control circuit 2 generates control commands two times per period, with the output of the SE 5 information

updated twice per period, i.e. the speed in this case is equal to half the period of the measured signal, and the output signal is proportional to the amplitude value of the measured component with an accuracy determined by the accuracy of the control commands.

The proposed method of measuring active and reactive components

current allows measurement with high accuracy and maximum speed.

Claims (2)

1. A method of measuring the active and reactive components of the current, based on the use of signals that are proportional to .-. Instantaneous values of voltage and current in a controlled circuit, characterized in that, for the purpose of increase, accuracy and measurement speed, they fix the level of total current at the time when the value of this level is equal to the amplitude value of the measured component, and the fixing time is determined once per period over the phase of the rectangular reference signal generated.:. of the measured voltage and network. . 2-. A method according to claim 1, characterized in that, in order to further increase the speed of the test, said fixation of the total current level is made an additional second time in the period, and the result of the measurement is inverted. Sources of information taken into account in the examination 1. USSR Author's Certificate No. 705354, cl. G 01 R 19/06. 2.Maryuta A.N., Voronov V.A. Active current and power factor sensors. - Instruments and systems of control, 1973, 1, q. 17-18.